An increasing number of portable electronic products are available today which operate on a battery source powering the device. These products include such things as cellular telephones, portable radios, pagers and voice recorders which are conveniently mobile and operate using rechargeable batteries. Many different battery chemistries have been used for many years which meet the need for recharging capability. Probably the most popular chemistries include nickel cadmium and nickel metal hydride. A relatively new chemistry, however, generally referred to as lithium ion, enables a cell to be recharged while offering many advantages over other types of rechargeable cells. These benefits primarily are directed to low weight and overall size with a high energy density. One unique factor to be considered when using a lithium ion cell is its charging scheme. A lithium ion cell is not charged in the same manner as cells using a nickel chemistry.
Nickel-cadmium and nickel metal hydride cells are typically charged using a rapid charge by applying a constant current until a certain event occurs. This event may be coupled to the cell reaching a predetermined high voltage, decreasing to a predetermined low voltage, or an increase in the cell's temperature. This is in contrast with the lithium ion cell which requires a two step charging process to achieve optimum performance. The first step of this process provides that the battery charger apply a constant current level while the cell's voltage remains below a predetermined threshold. Once the voltage increases to that threshold, the second step insures the battery charger is held at the threshold voltage allowing the current to decrease. Once the current decreases sufficiently to a desired level, the lithium ion cell is fully recharged.
This two step process presents a problem when considering charging lithium ion cells in a charger designed for nickel systems. Generally, nickel system chargers apply only a constant current which allows the voltage of the cells to rise unimpeded. The voltage may rise to any level provided the battery does not become too hot, i.e. increase to a undesired and dangerous level. Once the nickel system battery becomes hot, the charger detects this state and switches from the rapid high current charge to a value of approximately 5-10% that of the rapid current value.
Certain lithium ion cells are sensitive to current levels, and cannot be charged at the full rate typically provided by a nickel system charger. Accordingly, battery packs using such cells, or similar types, could not be made retrofittable in the absence of a means by which to control the charging current. One way to accomplish this is to have the charger change modes quickly, so that it only charges at the initial high rate for a brief period, and then switches to the lower current mode. This lower current mode is generally referred to as a trickle current or trickle charge, and is acceptable for charging smaller capacity lithium ion cells sensitive to overcurrent conditions.
Hence, the charging scheme offered by current nickel system chargers will not properly charge a lithium ion cell. Should a lithium ion cell be placed or forced into the nickel system charger the result could be potentially damaging since the lithium ion cell could quickly overheat.
Therefore, the need exists for a battery charging circuit or system which can be retrofitted to the control circuitry of an existing lithium ion cell allowing the cell to safely use a nickel system charger such that the charger does not provide an excessive current level for any extended period of time.